Controlled particle size distribution

ABSTRACT

Provided is a collection of polymer beads, wherein the volume distribution expressed as a function of bead diameter comprises
         (A1) a first peak having a maximum of PM1 at diameter PD1 and having full width at half maximum of 75 μm or less,   (A2) a second peak having a maximum of PM2 at diameter PD2 and having full width at half maximum of 75 μm or less; and   (B1) a valley having a minimum value of VM1 at a diameter between PD1 and PD2, wherein VM1 is less than 0.25*PM1 and VM1 is also less than 0.25*PM2.
 
wherein the polymer beads comprise polymerized units of one or more monofunctional vinyl monomer and one or more multifunctional vinyl monomer.

When considering a collection of polymer beads, it is important toconsider the distribution of particle sizes in the collection. Polymerbeads produced by aqueous suspension polymerization often have adistribution of particles sizes that is unimodal. That is, when theoccurrence of particles is plotted versus diameter, only a singlesignificant peak is observed. In such cases, the peak is oftenapproximated by a Gaussian distribution, in which the occurrence as afunction of diameter (d) is proportional to the functionexp(−(d−μ)²/(2σ²)), where μ is the mean and σ is the standard deviation.Such distributions of particle size are also characterized by theuniformity coefficient (“UC,” defined below). Unimodal distributionshaving UC greater than 1.5 are referred to herein as “typical Gaussian,”while unimodal distributions having UC less than 1.3 are referred toherein as “uniform.”

A drawback associated with a typical Gaussian distribution is thattypical Gaussian distributions of polymer particles made in suspensionpolymerization normally contain undesirable levels of very small andvery large particles. Thus the typical Gaussian collection of particlesmust be sifted to remove both the very large particles and the verysmall particles from the collection, and the sifting process reduces theyield of polymer particle production. Often, the loss of polymerparticles due to sifting of a typical Gaussian collection of particlesis 20 to 30% by weight. A drawback associated with a uniformdistribution is that, in a collection of spherical particles that areall the same size, the particles do not pack efficiently to fill avolume of space. It is desired to provide a collection of polymerparticles, and a method of making those particles, that provide one ormore of the following benefits: there is small or no loss of productionyield due to sifting, and/or the packing efficiency of the particles isgood. It is also desired to provide polymer particles that are useful asion exchange resins and/or as adsorbent resins.

One method of making particles is described in U.S. Pat. No. 7,727,555,which describes a method of making particles that involves forming astream of a mixture including a first material and a second material(for example, a polymer and a gelling precursor), exposing the stream tovibration, and treating the stream to form particles including the firstmaterial. It is desired to provide a process of making particles that isconsistent with aqueous suspension polymerization, and it is desired toprovide a collection of particles suitable as ion exchange resin and/oras adsorbent resins.

The following is a statement of the invention.

A first aspect of the present invention is a method of making acollection of polymer beads comprising

-   -   (a1) forming a first collection of monomer droplets in an        aqueous medium in a container, wherein the first collection of        monomer droplets has volume-average diameter DAV1 and has        uniformity coefficient less than 1.3;    -   (a2) forming a second collection of monomer droplets in the        aqueous medium in the container, wherein the second collection        of monomer droplets has volume-average diameter DAV2 and has        uniformity coefficient less than 1.3, wherein DAV1 and DAV2        differ by 10 μm or more;        wherein either step (a1) and step (a2) are performed        simultaneously; or step (a2) is performed after step (a1), while        the first collection of monomer droplets remains in the        container; or a combination thereof; and    -   (b) after steps (a1) and (a2), polymerizing the monomer droplets        by suspension polymerization to form the polymer beads.

A second aspect of the present invention is a collection of polymerbeads, wherein the volume distribution expressed as a function of beaddiameter comprises

-   -   (A1) a first peak having a maximum of PM1 at diameter PD1 and        having full width at half maximum of 75 μm or less,    -   (A2) a second peak having a maximum of PM2 at diameter PD2 and        having full width at half maximum of 75 μm or less; and    -   (B1) a valley having a minimum value of VM1 at a diameter        between PD1 and PD2, wherein VM1 is less than 0.25*PM1 and VM1        is also less than 0.25*PM2.        wherein the polymer beads comprise polymerized units of one or        more monofunctional vinyl monomer and one or more        multifunctional vinyl monomer.

The following is a brief description of the drawings.

FIG. 1 shows an example of a distribution of particle diameters of acollection of polymer beads.

FIG. 2 shows an overlay of the volume distributions of diameters forfour different hypothetical collections of monomer droplets.

FIG. 3 shows the distribution resulting from adding together the fourdistributions shown in FIG. 2.

FIG. 4 is the measured volume distribution of diameters of thecollection of polymer beads made in Example 1 below.

FIG. 5 shows five peaks corresponding to the volume distributions offive hypothetical collections of monomer droplets and the sum of thosefive peaks.

FIG. 6 shows the measured volume distribution of polymer beads producedin Example 3

The following is a detailed description of the invention.

As used herein, the following terms have the designated definitions,unless the context clearly indicates otherwise.

A “polymer,” as used herein, is a relatively large molecule made up ofthe reaction products of smaller chemical repeat units. Polymers mayhave structures that are linear, branched, star shaped, looped,hyperbranched, crosslinked, or a combination thereof polymers may have asingle type of repeat unit (“homopolymers”) or they may have more thanone type of repeat unit (“copolymers”). Copolymers may have the varioustypes of repeat units arranged randomly, in sequence, in blocks, inother arrangements, or in any mixture or combination thereof.

Vinyl monomers have the structure

where each of R¹, R², R³, and R⁴ is, independently, a hydrogen, ahalogen, an aliphatic group (such as, for example, an alkyl group), asubstituted aliphatic group, an aryl group, a substituted aryl group,another substituted or unsubstituted organic group, or any combinationthereof. Vinyl monomers are capable of free radical polymerization toform polymers. Some vinyl monomers have one or more polymerizablecarbon-carbon double bonds incorporated into one or more of R¹, R², R³,and R⁴; such vinyl monomers are known herein as multifunctional vinylmonomers. Vinyl monomers with exactly one polymerizable carbon-carbondouble bond are known herein as monofunctional vinyl monomers.

Styrenic monomers are vinyl monomers in which each of R¹, R², and R³ ishydrogen and —R⁴ has the structure

where each of R⁵, R⁶, R⁷, R⁸, and R⁹ is, independently, a hydrogen, ahalogen, an aliphatic group (such as, for example, an alkyl group or avinyl group), a substituted aliphatic group, an aryl group, asubstituted aryl group, another substituted or unsubstituted organicgroup, or any combination thereof.

Acrylic monomers are vinyl monomers in which each of R¹ and R² ishydrogen; R³ is either hydrogen or methyl; and —R⁴ has one of thefollowing structures:

where each of R¹¹, R¹², and R¹⁴ is, independently, hydrogen, a C₁ to C₁₄alkyl group, or a substituted C₁ to C₁₄ alkyl group.

A reaction among monomers to form one or more polymers is referred toherein as a polymerization process. The residue of a monomer after apolymerization process has taken place is known herein as a polymerizedunit of that monomer.

Polymer beads are individual particles, each containing 50% or more byweight of polymer. Beads are in the solid state at 23° C. Beads havevolume-average diameter of 1 μm or greater. If a particle is notspherical, the diameter of the particle is taken herein to be thediameter of an imaginary sphere that has the same volume as theparticle. How close a particle is to perfect spherical shape is assessedby first characterizing the three mechanical principal axes of theparticle, a (the longest principal axis), b, and c (the shortestprincipal axis). A particle is considered herein to spherical or nearlyspherical if the quotient (b/a) is 0.667 or greater and the quotient(c/b) is 0.667 or greater. As used herein the indicator of sphericity(Ψ) is calculated as follows:

$\Psi = \sqrt[3]{{bc}\text{/}a^{2}}$

As used herein, a chemical group is said herein to be “substituted” if asubstituent (that is, an atom or chemical group) is attached. Suitablesubstituents include, for example, alkyl groups, alkynyl groups, arylgroups, halogen atoms, nitrogen-containing groups including aminegroups, oxygen-containing groups including carboxyl groups,sulfur-containing groups including sulfonic acid groups, nitrile groups,and combinations thereof.

Droplets are individual particles of material in the liquid state.Droplets are dispersed in a continuous fluid medium, either liquid orgas. Droplets have volume-average diameter of 1 μm or higher. Acontinuous fluid medium is an “aqueous medium” if the continuous fluidmedium contains 50% or more water by weight based on the weight of thecontinuous fluid medium. Substances dissolved into solution areconsidered herein to be part of the continuous fluid medium. Substancesthat are present as particles dispersed in the continuous fluid mediumare not considered herein to be part of the continuous fluid medium.

A collection of particles is characterized by the volume-baseddistribution of diameters. The parameter DAV is the volume-averagediameter of the collection of particles. The parameter D60 denotes thevalue of a diameter that has the property that exactly 60% of thecollection of the particles, by volume, have diameter less than or equalto D60. The parameter D10 denotes the value of a diameter that has theproperty that exactly 10% of the collection of the particles, by volume,have diameter less than or equal to D10. The parameter “uniformitycoefficient” (abbreviated “UC”) is UC=D60/D10. The parameter D80 denotesthe value of a diameter that has the property that exactly 80% of thecollection of the particles, by volume, have diameter less than or equalto D80.

A collection of particles is also characterized by the harmonic meandiameter (HMD), which is defined as

${HMD} = \frac{N}{\sum\limits_{i = 1}^{N}\; \left( \frac{1}{d_{i}} \right)}$

Two numbers are said herein to “differ” by a specific amount if theabsolute value of the difference between the two numbers is thatspecific amount.

As used herein, an initiator is a molecule that is stable at ambientconditions but that is capable under certain conditions of producing oneor more fragments that bears a free radical, and that fragment iscapable of interacting with a monomer to start a free radicalpolymerization process. The conditions that cause production of afragment bearing a free radical include, for example, elevatedtemperature, participation in an oxidation-reduction reaction, exposureto ultraviolet and/or ionizing radiation, or a combination thereof.

The present invention involves a method of making polymer beads. Themethod involves providing a first collection of monomer droplets in anaqueous medium. The aqueous medium preferably contains water in anamount of 75% or more; more preferably 85% or more. The first collectionof particles has volume average diameter of DAV1. Preferably, DAV1 is100 μm or greater; more preferably 200 μm or greater. Preferably, DAV1is 1,000 μm or lower; more preferably 600 μm or lower. the firstcollection of particles has uniformity coefficient (UC) of 1.3 or less;more preferably 1.25 or less; more preferably 1.2 or less; morepreferably 1.15 or less; more preferably 1.1 or less.

The first collection of monomer droplets contains monomer. Preferably,the amount of monomer in the droplets is 50% or more by weight, based onthe weight of the droplets; more preferably 75% or more; more preferably85% or more; more preferably 95% or more. Preferred monomers are vinylmonomers. Preferably the amount of vinyl monomer in the first collectionof monomer droplets, by weight based on the weight of all monomers, is75% or more; more preferably 85% or more; more preferably 95% or more;more preferably 99% or more. Preferred monomers are styrenic monomersand acrylic monomers. Preferably the sum of the amount of styrenicmonomers plus the amount of acrylic monomers is, by weight based on theweight of all monomers, 75% or more; more preferably 85% or more; morepreferably 95% or more; more preferably 99% or more. Among styrenicmonomers, preferred are styrene and divinylbenzene. Among acrylicmonomers, preferred are acrylonitrile and methyl acrylate.

In the first collection of monomer droplets, preferably the dropletscontain one or more monofunctional vinyl monomer. Preferably the amountof monofunctional vinyl monomer is, by weight based on the weight of allmonomers, 25% or more; more preferably 50% or more; more preferably 75%or more. Preferably the amount of monofunctional vinyl monomer is, byweight based on the weight of all monomers, 99.5% or less. Preferredmonofunctional vinyl monomers are styrene, ethylbenzene, methylacrylate, acrylonitrile, and mixture thereof; more preferably styrene,ethylbenzene, and mixtures thereof.

In the first collection of monomer droplets, preferably the dropletscontain one or more multifunctional vinyl monomer. Preferably the amountof multifunctional vinyl monomer is, by weight based on the weight ofall monomers, 1% or more; more preferably 2% or more; more preferably 4%or more; more preferably 5% or more. The amount of multifunctional vinylmonomer is, by weight based on the weight of all monomers, 100% or less;preferably 99.5% or less; more preferably 50% or less; more preferably30% or less; more preferably 25% or less; more preferably 15% or less.Preferred multifunctional monomer is divinylbenzene.

In the first collection of monomer droplets, preferably the dropletscontain one or more initiator. Preferred initiators have solubility in100 mL of water at 25° C. of 1 gram or less; more preferably 0.5 gram orless; more preferably 0.2 gram or less; more preferably 0.1 gram orless. Preferred initiators are peroxygen initiators. Preferably, theamount of initiator in the droplets is, by weight based on the weight ofthe droplets, 0.05% or more; more preferably 0.1% or more. Preferably,the amount of initiator in the droplets is, by weight based on theweight of the droplets, 10% or less; more preferably 5% or less.

In the first collection of monomer droplets, the composition of thedroplets may be characterized by the sum of the amounts of all monomersand all initiators. Preferably, the sum of the weights of all monomersand all initiators is, based on the weight of the droplets, 75% or more;more preferably 85% or more. Preferably, the sum of the weights of allmonomers and all initiators is, 100% or less.

Preferably, the first collection of monomer droplets has volume averagesphericity Ψ of 0.7 to 1.0; more preferably 0.8 to 1.0; more preferably0.9 to 1.0.

The first collection of monomer droplets may be made by any method. Apreferred method is by vibratory excitation of a laminar jet of a liquidthat contains monomer flowing into a continuous aqueous medium. Asuitable method of performing such a method is described in U.S. Pat.No. 4,444,961. The suitable and preferred compositions for the liquidthat contains monomer are the same as described above for the suitableand preferred compositions of monomer droplets.

The method of the present invention also involves forming a secondcollection of monomer droplets. The suitable and preferred composition,distribution (including DAV and UC), and method of making the secondcollection of monomer droplets are all the same as those described abovefor the first collection of monomer droplets. These characteristics ofthe second collection of monomer droplets are chosen independently ofthe characteristics of the first collection of monomer droplets. Thecomposition of the second collection of monomer droplets may be the sameas or different from the composition of the first collection of monomerdroplets; preferably the composition of the second collection of monomerdroplets has the same composition as the first collection of monomerdroplets.

The second collection of monomer droplets has a volume-average diameterDAV2, which is different from DAV1. The quantities DAV1 and DAV2 differby 10 μm or more. That is, the absolute value of (DAV1−DAV2) is 10 μm ormore. Preferably DAV1 and DAV2 differ by 25 μm or more; more preferably50 μm or more; more preferably 75 μm or more. DAV2 may be larger orsmaller than DAV1.

In the practice of the present invention, the second collection ofmonomer droplets is made either under condition (I) or under condition(II) or under a combination of conditions (I) and (II). In condition(I), the first and second collections of monomer droplets are madesimultaneously. One way of making the first and second collections ofmonomer drops simultaneously (i.e., under condition (I)) would be to usethe method described in U.S. Pat. No. 4,444,961 using two separate feedsthrough two separate jets, discharging monomer droplets into the sameaqueous medium. The feed through one jet could be adjusted to produce acollection of monomer droplets having volume-average diameter DAV1 andhaving UC of 1.3 or less. The feed through the other jet could beadjusted to produce a collection of monomer droplets havingvolume-average diameter DAV2 and having UC of 1.3 or less, where DAV1and DAV2 differed by 10 μm or more.

In condition (II), the second collection of monomer droplets is madeafter the first collection of monomer droplets, and the secondcollection of monomer droplets is formed in the presence of the firstcollection of monomer droplets. For example, a first collection ofmonomer droplets could be made by using the method described in U.S.Pat. No. 4,444,961 to produce a collection of monomer droplets in anaqueous medium having DAV1 in an aqueous medium. Then, using the samefeed and the same jet, the conditions of jetting could be changed toproduce a collection of monomer droplets having DAV2, different fromDAV1 by 10 μm or more, and the jet could discharge the second collectionof monomer droplets into the aqueous medium that already contained thefirst collection of monomer droplets.

A variety of combinations of condition (I) and condition (II) areenvisioned. For one example, the method in U.S. Pat. No. 4,444,961 couldbe used, and the jetting conditions that produce the first collection ofmonomer droplets could be alternated with the jetting conditions thatproduce the second collection of monomer droplets. For another example,production of the first collection of monomer droplets could be begun,and at a later time, prior to the completion of the production of thefirst collection of monomer droplets, a separate feed with a separatejet, using different conditions but discharging into the same aqueousmedium, could be begun and operated for a time to simultaneously producemonomer droplets that belong to the first and second collections ofmonomer droplets.

Preferably the method of the present invention also involves forming oneor more additional collections of monomer droplets. The suitable andpreferred compositions, distributions, and methods of making are thesame as those described above for the first and second collections ofmonomer droplets. These characteristics of each additional collection ofmonomer droplets are chosen independently of the characteristics of theother collections of monomer droplets. The compositions of the variouscollections of monomer droplets may be the same as each other ordifferent from each other. Preferably the compositions of the variouscollections of monomer droplets are all the same.

Preferably each additional collection of monomer droplets has DAV thatis different by 10 μm or more from the DAV of one or more of the othercollections of monomer droplets that are present.

Preferably each additional collection of monomer droplets is made eithersimultaneously with one or more of the other collections of monomerdroplets or else is made in the presence of one or more of the othercollections of monomer droplets or a combination thereof. The discussionabove regarding condition (I) and condition (II) preferably apply toeach pair of collections of monomer droplets. In one embodiment, forexample, all the collections of monomer droplets are made in sequence,one after the other. In another embodiment, for example, all thecollections of monomer droplets are made simultaneously.

The monomer droplets are present in an aqueous medium. Preferably theaqueous medium contains one or more suspending agent. Preferredsuspending agents are gelatins, polyvinylalcohols, starches, polyacrylicacids, salts of polyacrylic acids; magnesium silicate; cellulose ethers;and mixtures thereof. More preferred are gelatin, polyvinyl alcohol,carboxymethylcellulose, and mixtures thereof. Preferably, the amount ofsuspending agent in the aqueous medium is, by weight based on the weightof the aqueous medium, 0.05% or more; more preferably 0.075% or more.Preferably, the amount of suspending agent in the aqueous medium is, byweight based on the weight of the aqueous medium, is 5% or less; morepreferably 3% or less.

When all the collections of monomer droplets have been made, and all thecollections of monomer droplets are present in the same aqueous medium,the resulting composition is known herein as the “final compositecollection of monomer droplets.” Preferably the total amount of allmonomer droplets is, by weight based on the total weight of the finalcomposite collection of monomer droplets, 5% or more; more preferably10% or more; more preferably 20% or more; more preferably 30% or more.Preferably the total amount of all monomer droplets is, by weight basedon the total weight of the final composite collection of monomerdroplets, 65% or less; more preferably 60% or less; more preferably 55%or less.

Preferably, the volume distribution of diameters of the final compositecollection of monomer droplets has D10 of 200 μm or higher; morepreferably 300 μm or higher. Preferably, the volume distribution ofdiameters of the final composite collection of monomer droplets has D80of 1,000 μm or smaller; more preferably 900 μm or smaller; morepreferably 800 μm or smaller.

The final composite collection of monomer droplets is subjected topolymerization conditions. The nature of the polymerization conditionsdepends in part on the nature of the initiator that is used.Polymerization conditions involve conditions in which the initiatorforms sufficient free radicals to initiate the polymerization process.For example, when a thermal initiator is used, polymerization conditionsinvolve establishing a temperature above 25° C. that is high enough fora significant fraction of the initiator molecules to decompose to formfree radicals. For another example, if a photoinitiator is used,polymerization conditions involve exposing the initiator to radiation ofsufficiently low wavelength and of sufficiently high intensity for asignificant fraction of the initiator molecules to decompose to formfree radicals. For another example, when the initiator is a redoxinitiator, polymerization conditions involve the presence ofsufficiently high concentration of both the oxidant and the reductantsuch that a significant number of free radicals are produced.Preferably, a thermal initiator is used. Preferably, polymerizationconditions involve temperature of 65° C. or higher; more preferably 75°C. or higher.

After the polymerization process is complete, the resulting compositionis preferably brought to ambient temperature (approximately 23° C.).Preferably, water and water-soluble compounds are separated from thepolymer beads formed in the polymerization process. After removal ofwater, the polymer beads preferably contain water in the amount, byweight based on the total weight of the polymer beads, of 20% or less;more preferably 15% or less; more preferably 10% or less; morepreferably 7% or less. After removal of water, the polymer beadspreferably contain water in the amount, by weight based on the totalweight of the polymer beads, of 1% or more; more preferably 2% or more;more preferably 3% or more.

The collection of polymer beads is preferably subjected to a sieveprocess to remove very small and very large particles. Preferably thecollection of polymer beads is passed through a sieve having mesh number14 or higher; 16 or higher; more preferably 18 or higher; morepreferably 20 or higher. Preferably the collection of polymer beads isretained on a sieve having mesh number 100 or lower; more preferably 80or lower; more preferably 70 or lower; more preferably 60 or lower.

Preferably, the amount of material lost in the sieving process, byweight based on the weight of the collection of polymer beads prior tosieving, is 25% or less; more preferably 20% or less; more preferably15% or less.

The polymerization conditions are chosen to promote suspensionpolymerization. In suspension polymerization, the monomer in the monomerdroplets polymerizes to form polymer beads. The resulting collection ofmonomer beads may be characterized by the distribution, on a volumebasis, of the bead diameters. An example of such a distribution is shownin FIG. 1.

The second aspect of the present invention involves a collection ofpolymer beads, where the collection has certain characteristics. Thecollection of polymer beads may be made by any method. The preferredmethod of making the collection of polymer beads is by methods of thefirst aspect of the present invention.

The volume distribution of the bead diameters shows two or more peaks.The distribution has a first peak and a second peak that meet thefollowing criteria. The first peak has a maximum value of PM1, whichoccurs at diameter value of PD1. The full width at half maximum of thefirst peak is 75 μm or less. The second peak has a maximum value of PM2,which occurs at diameter value of PD2. The full width at half maximum ofthe second peak is 75 μm or less. PD1 and PD2 differ by 25 μm or more.The volume distribution of the polymer bead diameters shows a valleythat has a minimum value of VM1 that occurs at diameter value that liesbetween PD1 and PD2. The value VM1 is less than 0.25*PM1, and the valueVM1 is also less than 0.25*PM2. FIG. 1 illustrates the assignment ofPD1, PM1, PD2, PM2, and VM1 to features on a volume distribution ofparticle diameters.

Preferably, the full width at half maximum of the first peak is 60 μm orless. Preferably, the full width at half maximum of the second peak is60 μm or less.

The label “first peak” and “second peak” may be applied to any pair ofpeaks that meet the above criteria, including the existence of therequired valley between the peaks. PD1 may be larger or smaller thanPD2. PM1 may be smaller or larger than PM2. Additional peaks may appearbetween the first peak and the second peak, as long as all the abovecriteria are met, including the existence of the required valley.Preferably, the volume distribution of the polymer bead diameterscontains three or more peaks, and one or more sets of three of the peakscan be identified, such that within each set of three peaks, each pairof two peaks qualifies as a first peak and a second peak as definedabove.

In order for a pair of peaks to qualify as a first peak and a secondpeak as defined above, there must be a valley as described above betweenthe two peaks. If three or more peaks are present, a single valley couldqualify as the required valley between more than one pair of peaks. Toillustrate this point, reference is made to FIG. 4. Three peaks arepresent, located at 400 μm, 510 μm, and 610 μm. The pair of peaks at 400μm and 510 μm qualify as first and second peaks because they have therequired characteristics, including the valley between them at 450 μm.Similarly, the pair of peaks at 400 μm and 610 μm also qualify as firstand second peaks because they have the required characteristics,including the valley at 450 μm that lies between them. Thus the valleyat 450 μm can serve as the required valley in two different pairs ofpeaks that qualify as first and second peaks.

When the collection of polymer beads is made by the method of the firstaspect of the present invention, it is expected that each collection ofmonomer droplets will become polymer beads that form a peak in thevolume distribution of diameters in the collection of polymer beads. Itis also contemplated that when two or more collections of monomerdroplets have volume-average diameters that are close to each other,those two collections of monomer droplets will, after polymerization,contribute to a feature in the volume distribution of diameters in thecollection of polymer beads that may be a single peak that qualifies asa “first peak,” or a single peak that has full width at half maximum ofmore than 75 μm, or a feature that is not a single peak. Regardless ofhow the collections of monomer droplets do or do not combine to formfeatures of the volume distribution of diameters in the collection ofpolymer beads, that distribution will have, possibly among otherfeatures, the first peak and the second peak described above.

Preferably, the polymer beads in the collection of polymer beads allhave the same composition. Preferably the amount of polymer in thepolymer beads is, by weight based on the weight of polymer beads, 80% ormore; more preferably 90% or more; more preferably 95% or more.Preferably the polymer beads contain polymerized units of monomers thatare the same as the preferred types and amounts of monomers describedabove as preferred for the content of the collections of monomerdroplets.

In the volume distribution of polymer bead diameters, D10 is preferably200 μm or larger; more preferably 300 μm or larger. In the volumedistribution of polymer bead diameters, D80 is preferably 1,000 μm orsmaller; more preferably 900 μm or smaller; more preferably 800 μm orsmaller.

Preferably, the collection of polymer beads has volume averagesphericity Ψ of 0.7 to 1.0; more preferably 0.8 to 1.0; more preferably0.9 to 1.0; more preferably 0.95 to 1.0.

FIG. 1 shows a hypothetical distribution of polymer particles. Thevertical axis is an arbitrary scale that shows the relative amount, byvolume, of particles at each diameter. The peaks and valleys are markedto illustrate how the parameters PM1, PD1, PM2, PD2, and VM1 aredetermined.

FIG. 2 shows the probability distributions of four separate collectionsof monomer droplets. The vertical axis is a probability distributionshowing the probable amount, on a volume basis, of monomer droplets ateach diameter. FIG. 3 shows the sum of the four distributions depictedin FIG. 2.

FIG. 4 shows a measured volume distribution of a collection of polymerbeads of the present invention. Details of this collection of polymerbeads are given in the Examples below. The vertical axis is an arbitraryscale that shows the relative amount, by volume, of particles at eachdiameter.

The following are examples of the present invention.

EXAMPLE 1 Preparation of a Multimodal Distribution

A suspension of droplets suspended in an aqueous medium was made usingthe procedure as described by U.S. Pat. No. 4,444,960 with an ensembleof particle sizes as described below.

A monomer mixture comprising 83.9 parts by weight styrene, 15.8 parts byweight divinylbenzene (mixture of 63% divinylbenzene and 27%ethylvinylbenzene), and 0.17 parts by weight of a peroxygen initiatorwas metered into a monomer reservoir, through the openings in an orificeplate, which plate has two openings (the size of which are specified inthe table below), for a total of 97 minutes at the four different,sequential, conditions as listed in the table below. The Reynolds numberand Strouhal number are defined and used as in U.S. Pat. No. 4,444,960.

Collection of Orifice Targeted Reynolds Strouhal monomer Time sizeDroplet Flow rate Number Number droplets (min) (um) Size (um) (ml/min)(Re) (St) A 20 199 447 6.5 313 0.86 B 30 199 526 6.5 313 0.53 C 30 199563 6.5 313 0.43 D 17 234 662 7.6 313 0.43

The first 20 minutes of operation produced collection of monomerdroplets “A.” Then, only the Strouhal number was changed, and operationcontinued for 30 minutes to produce collection of monomer droplets “B”in the same container as collection of monomer droplets “A.” Then, onlythe Strouhal number was changed, and operation continued for another 30minutes to produce collection of monomer droplets “C” in the samecontainer as collections of monomer droplets “A” and “B.” Then themonomer flow was stopped, the 199 μm orifice plate was replaced by the234 μm orifice plate, and then the monomer flow was resumed and operatedfor 17 minutes to produce collection of monomer droplets “D” in the samecontainer as collections of monomer droplets “A” and “B” and “C.”

To break the monomer jets into uniformly sized droplets, the jets werevibratorily excited at different Strouhal Numbers, dependent on thedesired drop diameter. The resulting uniform sized droplets rose throughthe column which comprises a co-currently fed aqueous solution of 0.05weight percent of a carboxymethyl methyl cellulose. The monomer dropletsflowed from the upper end of the column into a polymerization reactorwith continuous agitation until a suspension comprising about 40 volumepercent of the unpolymerized monomer droplets, based on the volume ofmonomer and continuous phases, was obtained. Sufficient amounts ofadditional carboxymethyl methyl cellulose were added to thepolymerization reactor to make the total concentration of the suspendingagent about 0.15 percent based on the weight of the suspending medium.Sufficient amounts of sodium dichromate were added to the polymerizationreactor to make the total concentration about 0.13 percent based on thesuspending medium. The monomer was then polymerized by heating thereactor to about 81° C. for a period of about 7 hours followed byheating the reactor for an additional 1.5 hours at about 100° C. whileagitating the suspension at conditions which minimize the coalescence orfurther dispersion of the droplets. At the end of this period theresulting polymer beads were recovered, free of suspending medium, usingconventional filtration techniques and were subsequently dried. Thedried beads were screened by passing through a screen of mesh number 20and by being retained on a screen of mesh number 60. To measure thevolume distribution of diameter, the dried beads were mixed with waterto form a slurry, which was measured by optical image analysis using aFlowCam™ (Fluid Imaging Technologies, Inc.). The slurry flowed past acamera, which recorded images of the particles. The diameter of eachparticle was determined by taking the mean of 36 Feret diameters at 5°intervals. A multi-point calibration from 74 to 1,700 μm was used.Results are shown in FIG. 4. Yield was estimated at 91%.

FIG. 2 shows four peaks corresponding to the volume distributions offour hypothetical collections of monomer droplets. FIG. 3 shows the sumof the four peaks in FIG. 2. Thus FIG. 3 is a theoretical prediction ofthe distribution that would result if the four hypothetical collectionsof monomer droplets shown in FIG. 2 were present in the same container.

It is contemplated that when a collection of monomer droplets undergoessuspension polymerization, the monomer in each monomer dropletpolymerizes in that droplet, thus converting each monomer droplet to apolymer bead of approximately the same size. Therefore it iscontemplated that the volume distribution of polymer beads that resultsfrom a suspension polymerization process will be similar or identical tothe volume distribution of monomer droplets that existed prior to thestart of polymerization.

The conditions for collections of monomer droplets A, B, C, and Ddescribed above were chosen in an attempt to create four actualcollections of monomer droplets that would be well described by the fourpeaks shown in FIG. 2. It was then expected that the volume distributionof diameters of the final composite collection of monomer droplets (thatis, the collection of monomer droplets that was present in the containerat the conclusion of forming collection of monomer droplets D) would bewell described by FIG. 3. It was also expected that the polymer beadsobtained by suspension polymerization of the final composite collectionof monomer droplets would also be well described by FIG. 3.

The measured volume distribution of polymer beads produced of Example 1is shown in FIG. 4. It can be seen that the theoretical distribution inFIG. 3 matches the measured distribution in FIG. 4 very well.

EXAMPLE 2 Measurement of Packing Density

Packing density was measured by the following method. The “as received”resin in an amount of weight W was placed into a dry, tared graduatedcylinder. The resin sample was gently tapped to a constant volume V. Thepacking density was calculated as follows:

Packing Density (kg/m³)=1000×W (g)/V (ml)

A comparative sample (Comparative 2) was as follows. Resin beads ofpolymer composition 90 parts by weight styrene and 10 parts by weightdivinylbenzene (mixture of 63% divinylbenzene and 27%ethylvinylbenzene); harmonic mean diameter 490 μm, and uniformitycoefficient of 1.05.

The packing densities were as follows:

Sample Packing Density Example 1 650 kg/m³ (40.6 lb/ft³) Comparative 2632 kg/m³ (39.5 lb/ft³)

The Example polymer beads have higher packing density than theComparative polymer beads.

EXAMPLE 3 Preparation of Another Example Distribution

A suspension of droplets suspended in an aqueous medium was made using aprocedure similar to that described by U.S. Pat. No. 4,444,960.

A monomer mixture comprising 83.9 parts by weight styrene, 15.8 parts byweight divinylbenzene (mixture of 63% divinylbenzene and 27%ethylvinylbenzene), and 0.17 parts by weight of a peroxygen initiatorwas metered into a monomer reservoir, through the openings in an orificeplate, which plate has two openings, each having diameter 199 μm, for atotal of 1007 minutes at the five different, sequential, conditions aslisted in the table below. The Reynolds number and Strouhal number aredefined and used as in U.S. Pat. No. 4,444,960. For all five conditions,flow rate was 6.5 ml/min and the Reynolds number was 313.

Collection of Targeted Strouhal monomer Time Droplet Number droplets(min) Size (um) (St) A 20 447 0.86 B 20 474 0.73 C 20 500 0.62 D 20 5260.50 E 20 557 0.46

Each condition was maintained for 20 minutes, then only the Strouhalnumber was changed, and the next condition was maintained for 20 minutesto produce a new collection of monomer droplets in the same container asall the previous collections of monomer droplets.

To break the monomer jets into uniformly sized droplets, the jets werevibratorily excited at different Strouhal Numbers, dependent on thedesired drop diameter. The resulting uniform sized droplets rose throughthe column which comprises a co-currently fed aqueous solution ofpolyvinyl alcohol. The monomer droplets flowed from the upper end of thecolumn into a polymerization reactor with continuous agitation until asuspension comprising about 40 volume percent of the unpolymerizedmonomer droplets, based on the volume of monomer and continuous phases,was obtained. Sufficient amounts of additional carboxymethyl methylcellulose were added to the polymerization reactor to make the totalconcentration of the suspending agent about 0.15 percent based on theweight of the suspending medium. Sufficient amounts of sodium dichromatewere added to the polymerization reactor to make the total concentrationabout 0.13 percent based on the suspending medium. The monomer was thenpolymerized by heating the reactor to about 81° C. for a period of about7 hours followed by heating the reactor for an additional 1.5 hours atabout 100° C. while agitating the suspension at conditions whichminimize the coalescence or further dispersion of the droplets. At theend of this period the resulting polymer beads were recovered, free ofsuspending medium, using conventional filtration techniques and weresubsequently dried. The dried beads were screened by passing through ascreen of mesh number 20 and by being retained on a screen of meshnumber 60. To measure the volume distribution of diameter, the driedbeads were mixed with water to form a slurry, which was measured byoptical image analysis using a FlowCam™ (Fluid Imaging Technologies,Inc.). Results are shown in FIG. 6. Yield was estimated at 91%.

FIG. 5 shows five peaks corresponding to the volume distributions offive hypothetical collections of monomer droplets, displayed as dottedlines. FIG. 5 also shows the sum of those five peaks; the sum isdisplayed as a solid line. Thus the solid line in FIG. 5 is atheoretical prediction of the distribution that would result if the fivehypothetical collections of monomer droplets shown in the dotted linesof FIG. 5 were present in the same container.

It is contemplated that when a collection of monomer droplets undergoessuspension polymerization, the monomer in each monomer dropletpolymerizes in that droplet, thus converting each monomer droplet to apolymer bead of approximately the same size. Therefore it iscontemplated that the volume distribution of polymer beads that resultsfrom a suspension polymerization process will be similar or identical tothe volume distribution of monomer droplets that existed prior to thestart of polymerization.

The conditions for collections of monomer droplets A, B, C, D, and Edescribed above were chosen in an attempt to create five actualcollections of monomer droplets that would be well described by the fivepeaks shown in FIG. 5. It was then expected that the volume distributionof diameters of the final composite collection of monomer droplets (thatis, the collection of monomer droplets that was present in the containerat the conclusion of forming collection of monomer droplets E) would bewell described by FIG. 5. It was also expected that the polymer beadsobtained by suspension polymerization of the final composite collectionof monomer droplets would also be well described by FIG. 5.

The measured volume distribution of polymer beads produced of Example 3is shown in FIG. 6. It can be seen that the theoretical distribution inthe solid line of FIG. 5 matches the measured distribution in FIG. 6very well.

1. A collection of polymer beads, wherein the volume distributionexpressed as a function of bead diameter comprises (A1) a first peakhaving a maximum of PM1 at diameter PD1 and having full width at halfmaximum of 75 μm or less, (A2) a second peak having a maximum of PM2 atdiameter PD2 and having full width at half maximum of 75 μm or less; and(B1) a valley having a minimum value of VM1 at a diameter between PD1and PD2, wherein VM1 is less than 0.25*PM1 and VM1 is also less than0.25*PM2. wherein the polymer beads comprise polymerized units of one ormore monofunctional vinyl monomer and one or more multifunctional vinylmonomer.
 2. The collection of polymer beads of claim 1, wherein thepolymer beads comprise polymerized units of one or more monomer selectedfrom styrenic monomers, acrylic monomers, and mixtures thereof.
 3. Thecollection of polymer beads of claim 1, wherein the sum of the amount ofpolymerized units of styrenic monomers plus the amount of polymerizedunits of acrylic monomers is, by weight based on the weight of thepolymer beads, 75% or more; more preferably 85% or more; more preferably95% or more; more preferably 99% or more.
 4. The collection of polymerbeads of claim 1, wherein the volume distribution expressed as afunction of bead diameter additionally comprises (A3) a third peakhaving a maximum of PM3 at diameter PD3 and having full width at halfmaximum of 75 μm or less, (B2) a valley having a minimum value of VM2 ata diameter between PD1 and PD3, wherein VM2 is less than 0.25*PM1, andVM2 is also less than 0.25*PM3. (B3) a valley having a minimum value ofVM3 at a diameter between PD3 and PD2, wherein VM3 is less than0.25*PM3, and VM1 is also less than 0.25*PM2.